compiler: Fix sys_core_fold let optimization

Map variable was not covered and faulty optimization could occur.

Ex.
    t() ->
        M0 = id(#{ "a" => 1 }),
        #{ "a" := _ } = M0,
        M0#{ "a" := b }.

M0 was lost in let expression optimization.

2 files changed, 13 insertions, 2 deletions

diff --git a/lib/compiler/src/core_lib.erl b/lib/compiler/src/core_lib.erl
index f506901099..ed181e3baa 100644
--- a/lib/compiler/src/core_lib.erl
+++ b/lib/compiler/src/core_lib.erl
@@ -105,8 +105,8 @@ vu_expr(V, #c_cons{hd=H,tl=T}) ->
     vu_expr(V, H) orelse vu_expr(V, T);
 vu_expr(V, #c_tuple{es=Es}) ->
     vu_expr_list(V, Es);
-vu_expr(V, #c_map{es=Es}) ->
-    vu_expr_list(V, Es);
+vu_expr(V, #c_map{var=M,es=Es}) ->
+    vu_expr(V, M) orelse vu_expr_list(V, Es);
 vu_expr(V, #c_map_pair{key=Key,val=Val}) ->
     vu_expr_list(V, [Key,Val]);
 vu_expr(V, #c_binary{segments=Ss}) ->
diff --git a/lib/compiler/test/map_SUITE.erl b/lib/compiler/test/map_SUITE.erl
index dc880a7a9d..cad6ac6887 100644
--- a/lib/compiler/test/map_SUITE.erl
+++ b/lib/compiler/test/map_SUITE.erl
@@ -179,6 +179,17 @@ t_update_map_expressions(Config) when is_list(Config) ->
     #{ a :=42, b:=42, c:=42 } = (maps:from_list([{a,1},{b,2},{c,3}]))#{ a := 42, b := 42, c := 42 },
     #{ "a" :=1, "b":=42, "c":=42 } = (maps:from_list([{"a",1},{"b",2}]))#{ "b" := 42, "c" => 42 },
 
+    %% Test need to be in a fun.
+    %% This tests that let expr optimisation in sys_core_fold
+    %% covers maps correctly.
+    F = fun() ->
+	    M0 = id(#{ "a" => [1,2,3] }),
+	    #{ "a" := _ } = M0,
+	    M0#{ "a" := b }
+    end,
+
+    #{ "a" := b } = F(),
+
     %% Error cases, FIXME: should be 'badmap'?
     {'EXIT',{badarg,_}} = (catch (id(<<>>))#{ a := 42, b => 2 }),
     {'EXIT',{badarg,_}} = (catch (id([]))#{ a := 42, b => 2 }),